The present invention relates to a method of recovering lithium carbonate from brines by removing magnesium as magnesium hydroxide.
Lithium metal has many industrial uses including, e.g., employing a blanket of the liquid metal for breeding purposes in nuclear fusion reactors in the nuclear power industry. Additionally, lithium metal is used in lightweight, compact primary and secondary lithium batteries for military and commercial applications and as a degasifier in the production of high-conductivity copper and bronze. Another use of this metal is in the synthesis of organometallic compounds for applications in the fields of rubber, plastics and medicines. Lithium metal is generally produced for such uses by electrolysis of an eutectic mixture of highly pure molten lithium chloride and potassium chloride. Lithium carbonate also finds many applications, particularly in the pharmaceutical industry.
Naturally occurring brines found, e.g., in the United States and Chile, contain reasonable concentrations of lithium, in the form of lithium chloride. These brines are viable reserves for lithium recovery. These brines also contain varying amounts of boron, calcium and other components. Some typical components of naturally occurring brines are identified in the Table below (all values in weight percent):
__________________________________________________________________________ Great Bonne- Salton Silver Salar de Atacama Dead Salt ville Sea Peak Brines Sea Lake Brine Brine Brine Chile Ocean Israel Utah Utah Calif Nevada 1 2 __________________________________________________________________________ Na 1.05 3.0 7.0 9.4 5.71 6.2 7.17 5.70 K 0.038 0.6 0.4 0.6 1.42 0.8 1.85 1.71 Mg 0.123 4.0 0.8 0.4 0.028 0.02 0.96 1.37 Li 0.0001 0.002 0.006 0.007 0.022 0.02 0.15 0.193 Ca 0.04 0.05 1.5 0.5 0.0 0.71 1.46 0.043 Cl 1.9 16.0 14.0 16.0 15.06 10.06 16.04 17.07 Br 0.0065 0.4 0.0 0.0 0.0 0.002 0.005 0.005 B 0.0004 0.003 0.007 0.007 0.039 0.005 0.04 0.04 Li/Mg 0.0008 0.0005 0.0075 0.0175 0.786 1.0 0.156 0.141 Li/K 0.0026 0.0033 0.015 0.0049 0.0155 0.016 0.081 0.113 Li/Ca 0.0025 0.0064 0.2 0.0583 0.0008 1.0 4.84 0.244 Li/B 0.25 0.6666 0.857 1.0 0.051 4.0 3.75 4.83 __________________________________________________________________________
Some of these brines have high concentrations of lithium and a low magnesium to lithium ratio, generally about 1:1 to 6:1, which allow for a simplified process of concentrating, purifying and recovering lithium chloride brine. Lithium carbonate is then obtained by treatment of the brine with soda ash.
The impurities, such as, magnesium, calcium, sodium, sulfate and boron present in lithium containing natural brines, should be minimized to produce a lithium carbonate product suitable for its intended use. Alkali and alkaline earth metals, such as sodium, calcium, and especially magnesium must be substantially removed, otherwise, they will report as contaminants. Simple technical means for their removal from the metal are not cost effective.
U.S. Pat. No. 4,261,960 discloses the removal of boron, as well as magnesium and sulfate, by treatment of the brine with an aqueous slurry of slaked lime and an aqueous solution of calcium chloride, followed by concentrating. In addition, it is known that magnesium can be removed from brine from the Salar de Atacama in Chile by concentrating the brine to contain approximately 4% lithium (hereinafter the 4% lithium process). The magnesium from this concentrated brine, e.g. concentrated to contain approximately 4.3% lithium and 3.5% magnesium, may be precipitated by raising the pH to 11 to yield magnesium hydroxide as a precipitate; however, the magnesium hydroxide forms a fine precipitate and is difficult to filter, rendering this process unsuitable for commercial practice.
In the 4% lithium brine, the magnesium to lithium weight ratio is about 0.83 (3.5% Mg, 4.2% Li). Magnesium, in the 4% lithium process, is partially removed as magnesium carbonate utilizing the carbonate ion present in the recycled spent brine. All the magnesium cannot be removed as the carbonate without precipitating lithium carbonate. Therefore, a second purification step is required whereby magnesium is precipitated as Mg(OH).sub.2.
In addition to controlling magnesium, calcium coming from the slaked lime must also be removed because calcium will report with magnesium directly to the lithium carbonate product. Recycling the mother liquor will remove up to 70% of the magnesium first as the carbonate. The remaining magnesium can then be removed by addition of a lime/soda ash reagent slurry. Thus, a two step process is required to remove magnesium from the 4% lithium brine. The two step process results in a greater yield of lithium than precipitating Mg(OH).sub.2 alone.
There is a need in the art for improved techniques allowing for improved yields of lithium carbonate.
The present invention addresses the aforementioned difficulties with prior art techniques by providing a simplified single step process for removing magnesium in a single step from brine saturated with respect to lithium and magnesium while recovering lithium carbonate from the brine. Mother liquor from the lithium carbonate precipitation reaction is recycled and reacted with brine to produce calcium carbonate and base to precipitate the magnesium hydroxide. A reagent of slaked lime and soda ash is then added to adjust the final pH prior to filtering the magnesium hydroxide and calcium carbonate solids. The lithium is then precipitated from the purified brine as lithium carbonate.
It is important to maintain the pH between 8.40 to 8.80, preferably between 8.55 and 8.75, during the magnesium hydroxide precipitation, i.e., when the limed mother liquor is mixed with brine.
The single step 6% lithium brine process of the present invention uses a single purification step whereby the magnesium in the plant feed brine is precipitated as Mg(OH).sub.2. The process results in greater yields of lithium carbonate than the aforementioned process.
The process of the present invention is described in more detail below.